The present invention relates to a novel optical recording medium from which extremely small recorded marks and pits having a size not exceeding 100 nm can be read out with stability to overcome the limitation in the prior art optical recording media for reading-out or reproduction of recorded signals.
As is well known, optical recording media are playing a leading role in the information-processing technology as a signal-recording means in the highly information-leading society in recent years in respect of the advantages that the optical recording medium is suitable for recording and reproduction of signals in an extremely high density and at a high speed.
Several classes of optical recording media of, in particular, the rewritable type have been developed and employed in the prior art including the magneto-optical recording media utilizing the interaction of light and magnetism called the Kerr effect or Faraday effect, and the phase change recording media utilizing the difference in an optical property, such as refractive index, transmissivity and reflectivity, caused by light between the amorphous phase and crystalline phase in a specific alloy composed of chalcogen elements.
On the other hand, optical recording media as a class of the write-once type utilizes the difference in the optical properties in the recording layer containing an organic dye which is susceptible to irreversible discoloration as a result of decomposition caused by heat in the areas irradiated with light.
In order to comply with the rapid progress of the highly information-leading society, in which further advancement is required for higher recording densities and higher velocity of recording and reproduction of signals in an optical recording medium, intensive investigations are now under way for the optical recording media including rewritable DVD-RAMs and write-once DVD-Rs.
Among the above mentioned various types of optical recording media, those of the phase change type are more promising for further increase of the recording density as a consequence of the characteristics of chalcogen alloys. When used in combination with a blue laser beam as the light for recording, for example, a recording density of as high as 15 gigabytes has been accomplished on a single surface of an optical recording disk with a 12-cm diameter (Preprints in Ninth Symposium on Phase Change Recording, 1997, page 94).
Further, a proposal is made in Japanese Journal of Applied Physics, volume 35 (1996), page 443, for an optical recording medium of the phase change type by utilizing the changes in the optical properties from the amorphous state of the recording layer as deposited to the crystalline state for further increase of the recording density. According to this report, crystalline marks of 60 to 200-nm diameters can be successfully formed in the optical recording medium by utilizing recording by optical near-field light with 60 nm as the smallest limit of the size of recording marks. In addition, a large activation energy is required in the optical recording medium of this type for the formation of crystalline marks of the GeSbTe alloy by the change from the as-deposit state.
Besides, a proposal is made in Japanese Journal of Applied Physics, volume 36 (1997), page 523, for the attempt of phase change type recording by utilizing an atomic force microscope in which a difference of electric charges is produced by the Schottky contact between the recording layer and the chromium-coated head of the atomic force microscope to accomplish recording of a recording mark of about 10-nm diameter. As a natural consequence of the use of the head in the atomic force microscope, this method as such is not suitable for reproduction of the recorded signals.
When high-density recording is carried out by using a near-field light or an atomic force, in particular, reproduction of recorded signals by these methods so far developed is demonstrated only under microscopes so that the reproduction of signals can never be accomplished at a high data transfer rate. This is because optical near-field intensity exponentially decreases with a propagation distance. This field cannot propagate to more than 100 nm. Therefore, the recording medium cannot be rotated to get an actual data-transfer rate of CD or DVD, otherwise the head soon makes crashes to the medium surface. This problem is held also in the use of an atomic force microscope. Namely, it is an extremely difficult matter to control such a short distance between the recording medium and the recording head under high-speed movement in a nanometer-order accuracy.